Planet earth is a giant magnet. Humans have used earth's magnetic fields as a way of navigating over great distances for hundreds of years. But in the last century scientists have discovered that igneous rocks (from volcanic lava) record a very important piece of information about earth's magnetic field at the time they are formed - where geomagnetic north and south poles were located at the time the lava cooled. In looking at rock samples around the world more closely the data they've collected has shown a pattern of regular polar reversals that has occurred throughout geologic history. The enormous forces at work inside of earth drive changes in the strength of the magnetic forces on its surface, so that over a very long time, the earth's geomagnetic poles trade places (see the animation at right, showing the changes in magnetic declination over the last 400 years).

By taking core samples of rock scientists can track a continent's travels over geologic time. They can also use the magnetic record to determine what the continents and sea floor positions were relative to each other. They've used their data to create maps and models of where continents were positioned over geologic time and there appeared some enormous drifting - lending more support to the theory of plate tectonics.